1. Field of the Invention
The present invention relates generally to orthopaedic surgical implant devices and, more particularly, to an implant apparatus for use in uniting a pair of bones or bone segments wherein each of the bones or segments includes a medullary cavity.
2. Discussion of the Prior Art
Arthrodesis of the knee, or knee fusion, is a surgical procedure usually performed as a method of salvaging a severely damaged knee which is unstable or which is painful and limited in motion, and has possibly had many previous surgeries. Knee fusion is obtained by techniques which immobilize the joint as rigidly as can be obtained after excising bone, if necessary, from the femur and tibia at their interface. Bone fracture healing is achieved in a similar manner by positioning two fractured bone segments in close contact with one another and immobilizing the bone until fusion of the segments is achieved.
Conventional techniques for knee fusion include the use of devices for external and internal fixation of the joint. An external fixation device which provides compression to the joint was introduced by J. A. Key and John Charnley. According to these methods, pins are inserted through the long bones above and below the joint to be fused. Thereafter, an external fixator or frame is applied to the pins and compression of the joint is obtained by securing the fixator or frame relative to the pins and pulling the pins toward one another.
Although devices used in carrying out this known technique have been successful in achieving fusion of a knee joint, several drawbacks exist which suggest the need for an improved technique and apparatus. For example, it is necessary in the known devices to employ a framework which is retained in a position external to the skin of the knee joint for a period of several weeks or months beyond surgery. Thus, it is difficult to protect against infections and the patient is constantly in a position of discomfort.
A further drawback to the use of conventional external techniques resides in the inability of such techniques to provide reliable restraint of the joined bones or segments in all dimensions so as to prevent any substantial movement therebetween. For example, according to the abovedescribed technique a framework is attached between a pair of pins in such a way as to restrict relative axial movement of the femur and tibia, but fails to provide any substantial torsional or lateral support to the joint.
Internal fixation for knee fusion is conventionally achieved by passing an intramedullary rod, known to fracture fixation, into the femur and then in retrograde fashion, into the tibia. The ends of the rod are then secured with bone screws. This method imparts immediate stability to the joint, even in the case of failed arthroplasty where little cancellous bone may remain when the prosthesis has been removed.
The bone screws used to secure an intramedullary rod in place are inserted in a transverse direction to the rod and extend through the bone cortex on either side thereof. Known techniques for installing these bone screws require the use of X-rays to locate holes in the intramedullary rod through which the screws are to be passed so that holes may be drilled in the bone in alignment with the holes in the rod. Thus, these techniques require that the patient and surgical team be exposed to X-rays, and may involve the use of trial and error in aligning the drill with the holes in the rod.
Another problem encountered with known knee fusion devices arises as a result of the resorption of old bone which occurs as a result of the bones natural ability to rearrange its structure in a pattern which provides maximum strength while using a minimum of material. When an implant device is affixed to a bone it alters the manner in which that bone carries a load. This alteration in the load-bearing capacity of the bone, while potentially offering short-term benefits to the patient, may result in a combination of resorption of old bone and generation of new bone such that the bone is no longer able to support the implant device or the patient's activity, and failure occurs.
In an attempt to minimize the affect of an implant device on bone resorption where intramedullary rod devices are employed, it is known to either unfasten the bone screws from one end of the rod or to completely remove the rod after fusion has occurred so that compression of the bone is permitted when the bone is loaded. However, both of these known techniques require the use of additional surgery and its attendant risks.
In order to achieve fusion of a knee joint or proper fixation of a fractured bone, no separation between the bones or segments being joined may be permitted and only minimal displacement of the bones or segments should occur. When such a relationship between the bones or segments is not achieved, fibrous tissue can form at the desired fusion sight causing pain to the patient and impeding fusion.